As densities for linear tape storage systems increase due to advancements in materials and storage schemes, precision alignment of the tape heads to data tracks is an increasingly challenging requirement for accurate recording and reading of stored data. At high track densities, alignment is affected by changes in tape width due to environmental conditions, such as temperature and humidity, and mechanical stress to the tape, which can cause the lateral spacing of the data tracks in a data band to shrink or expand. Generally, increasing the temperature or humidity will cause expansion of the tape width, and applying longitudinal stress to the tape will cause tension narrowing of the tape width. Changes to the lateral spacing of the data tracks may result in a misalignment of the tape head elements on the tape head assembly, such as read and write heads, with the data tracks on the tape. Depending on the latitudinal density of data tracks and the coefficients of thermal and hygroscopic expansion of the tape substrate material, the degree of expansion under certain environmental conditions may cause the lateral spacing of the data tracks under the read heads of the tape head assembly to expand to the point where not all data tracks can be read by the tape heads.
One method of adjusting for the effects of changes to tape width while recording and reading data is to change the relative spacing of the read and write elements with respect to the tape by adjusting the tape head azimuth angle. Another method is to change the longitudinal tension on the tape, causing a corresponding change to the lateral dimension of the tape. To facilitate this adjustment, a pair of servo tracks can be used to measure the physical width of the tape relative to that of the two servo elements.
Correct read and write element spacing relative to the data tracks in the presence of changing tape width may be maintained by a feedback control system. Such a control system may require both a method of determining the relative spacing, and an established reference value, or servo set point. One method of determining the relative spacing makes use of servo patterns written in the servo bands on either side of data bands during manufacturing. The servo patterns typically consist of magnetic transitions with two different azimuthal slopes, such as a chevron pattern, that are read by a pair of servo read heads located on the tape head. The servo read heads' lateral-position relative to a servo track is derived from the relative timing of pulses read by the servo read heads while reading the servo pattern. Reference signals for feedback control can then be chosen with respect to the spacing computed from the servo patterns. For example, the control system could adjust the azimuth angle of the tape head assembly or the tape tension such that both servo read heads track in the center of their respective servo patterns, and thus properly adjust the read head spacing relative to the data tracks. If the servo spacing changes while the tape is running in the tape drive unit, the servo feedback control system can compensate for these changes.
A number of factors can contribute to differences in servo pattern spacing. During manufacturing of the tape, the pattern width may be affected by the environmental conditions during the writing of the servo patterns. Further variation may be introduced by tolerances in constructing the servo writer head, which will also lead to differences in servo pattern spacing. Additionally, aging of the tape causes the servo pattern spacing to change over time. This change is typically non-uniform due to the differences in pack pressure, or how tightly wound the tape is, within the tape cartridge. For example, tape at the inner diameters of the cartridge typically become wider than tape at the outer diameters due to these differences in pack pressure.